[urcu.git] / README
1 Userspace RCU Implementation
2 by Mathieu Desnoyers and Paul E. McKenney
5 --------
7 make
8 #force 32-bit build with: make -f Makefile32
9 #force 64-bit build with: make -f Makefile64
10 make install
14 -----------------
16 Usage of all urcu libraries
18 * Define _LGPL_SOURCE (only) if your code is LGPL or GPL compatible
19 before including the urcu.h or urcu-qsbr.h header. If your application
20 is distributed under another license, function calls will be generated
21 instead of inlines, so your application can link with the library.
22 * Linking with one of the libraries below is always necessary even for
23 LGPL and GPL applications.
25 Usage of liburcu
27 * #include <urcu.h>
28 * Link the application with "-lurcu".
29 * This is the preferred version of the library, both in terms of speed
30 and flexibility. Requires a signal, typically SIGUSR1. Can be
31 overridden with -DSIGURCU by modifying Makefile.build.inc.
33 Usage of liburcu-mb
35 * #include <urcu.h>
36 * Compile any _LGPL_SOURCE code using this library with "-DURCU_MB".
37 * Link with "-lurcu-mb".
38 * This version of the urcu library does not need to
39 reserve a signal number. URCU_MB uses full memory barriers for
40 readers. This eliminates the need for signals but results in slower
41 reads.
43 Usage of liburcu-qsbr
45 * #include <urcu-qsbr.h>
46 * Link with "-lurcu-qsbr".
47 * The QSBR flavor of RCU needs to have each reader thread executing
48 rcu_quiescent_state() periodically to progress. rcu_thread_online()
49 and rcu_thread_offline() can be used to mark long periods for which
50 the threads are not active. It provides the fastest read-side at the
51 expense of more intrusiveness in the application code.
53 Usage of liburcu-bp
55 * #include <urcu-bp.h>
56 * Link with "-lurcu-bp".
57 * The BP library flavor stands for "bulletproof". It is specifically
58 designed to help tracing library to hook on applications without
59 requiring to modify these applications. urcu_init(),
60 rcu_register_thread() and rcu_unregister_thread() all become nops.
61 The state is dealt with by the library internally at the expense of
62 read-side and write-side performance.
64 Initialization
66 Each thread that has reader critical sections (that uses
67 rcu_read_lock()/rcu_read_unlock() must first register to the URCU
68 library. This is done by calling rcu_register_thread(). Unregistration
69 must be performed before exiting the thread by using
70 rcu_unregister_thread().
72 Reading
74 Reader critical sections must be protected by locating them between
75 calls to rcu_read_lock() and rcu_read_unlock(). Inside that lock,
76 rcu_dereference() may be called to read an RCU protected pointer.
78 Writing
80 rcu_assign_pointer() and rcu_xchg_pointer() may be called anywhere.
81 After, synchronize_rcu() must be called. When it returns, the old
82 values are not in usage anymore.
84 Usage of liburcu-defer
86 * #include <urcu-defer.h>
87 * Link with "-lurcu-defer"
88 * Provides call_rcu() primitive to enqueue delayed callbacks. Queued
89 callbacks are executed in batch periodically after a grace period.
90 Do _not_ use call_rcu() within a read-side critical section, because
91 it may call synchronize_rcu() if the thread queue is full.
93 Being careful with signals
95 The liburcu library uses signals internally. The signal handler is
96 registered with the SA_RESTART flag. However, these signals may cause
97 some non-restartable system calls to fail with errno = EINTR. Care
98 should be taken to restart system calls manually if they fail with this
99 error. A list of non-restartable system calls may be found in
100 signal(7). The liburcu-mb and liburcu-qsbr versions of the Userspace RCU
101 library do not require any signal.
103 Read-side critical sections are allowed in a signal handler with
104 liburcu and liburcu-mb. Be careful, however, to disable these signals
105 between thread creation and calls to rcu_register_thread(), because a
106 signal handler nesting on an unregistered thread would not be allowed to
107 call rcu_read_lock().
109 Read-side critical sections are _not_ allowed in a signal handler with
110 liburcu-qsbr, unless signals are disabled explicitly around each
111 rcu_quiescent_state() calls, when threads are put offline and around
112 calls to synchronize_rcu(). Even then, we do not recommend it.
114 Usage of DEBUG_RCU
116 DEBUG_RCU is used to add internal debugging self-checks to the
117 RCU library. This define adds a performance penalty when enabled.
118 Can be enabled by uncommenting the corresponding line in
119 Makefile.build.inc.
121 Usage of DEBUG_YIELD
123 DEBUG_YIELD is used to add random delays in the code for testing
124 purposes.
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